This is a tale of two parties. The first is the Green party in America, which has just selected consumer-rights advocate and maverick, Ralph Nader as its presidential candidate. Nader's support currently stands at 7% in the polls. That is not enough for Nader to make it into presidential debates, but more than enough for him to make a crucial difference between Democrat Al Gore and Republican George W Bush, who are running neck and neck.

The second is the Green party in Germany, which has just emerged from a divisive row at its congress over nuclear power. The government, in which it is a junior partner, had just reached a deal with the nuclear industry to phase out nuclear energy and shut down power stations, within 32 years. One of the party's co-leaders said she would resign if congress backed the deal; another said the coalition would collapse if he didn't. The congress supported it. The row continues. Meanwhile the party, which has seen its vote decline in the last 14 federal, state and European elections, is haemorrhaging support.

The first has no chance of power and is rising in popularity; the second is in power and finding its popularity dwindling. More intriguingly, those two conditions may well be interrelated. Limited evidence seems to suggest that there is an inverse relationship between a Green party's proximity to power and the number of people who want to back it. In other words the more it looks as though they actually might play a significant role in running a country, the less of a role people want them to play.

It is not a new dilemma. It has been an issue ever since the magical moment in 1983 when the German Greens wandered into the Bundestag wearing jeans and carrying potted plants. To what extent could they influence the political establishment and to what extent would the political establishment influence them? On the night of their election the late Petra Kelly, one of the key figures in the party during the 70s and 80s said: "Parliament is like the marketplace for us, or the building site, where we can spread the information. Parliament is not our goal, only part of our strategy." On hearing this, one of her colleagues grumbled that if she carried on talking like that: "We will slide into a minority sect faster than we can talk about it."

You can see the argument played out in Joschka Fischer's wardrobe. When he took the oath as the environment minister of Hesse in 1985 he did so in jeans and trainers. Now he is the German for eign minister and wears three-piece suits. Superficial? It would be if his politics had not changed just as dramatically. Fifteen years ago he was an opponent of Nato; last year he supported the bombing of Serbia.

This tension has always been depicted as a straightforward split between two factions - the "realos" and the "fundis". Realos believed that if you were going to stand in elections then you had to make some concessions to electoralism and if you wanted to work in coalitions you would have to compromise. The fundis declared that the very reason why they received their support was because they did not operate or look like other parties. As with most political constructs - old and New Labour, wet and dry Tories, modernisers and traditionalists - the putative split between realos and fundis was hopelessly simplistic and crude. However, it also had some basis in reality. The Greens have had to decide whether they can balance being a party of protest and a party of power and if so, at what price.

Nor is it a dilemma particular to the Greens. These calculations underpin the nature of all electoral politics. "What do we want to give up if we want to get in?" are the staple issues that have informed Labour's transformation over the past 15 years. Only when the Tories start asking themselves the same questions might they have a chance of being re-elected.

The answers necessarily change with the circumstances. Where the Greens are concerned, the circumstances at present are crucial. Greens have taken up positions in five European governments - France, Germany, the Netherlands, Italy and Finland. They also support center-left administrations in Sweden and New Zealand. And they have recently made a strong showing in Britain too. Thanks to the introduction of proportional representation they now have two MEPs, three GLA members and an MSP. The row between realos and fundis is no longer the stuff of internal bickering but the life and death of governments.

By any standards this is an impressive achievement. All the more so when one considers that, unlike other parties, the Greens have no rigid socio-economic base on which to rely. The American Greens may be strongest among the alternative lifestyles of California. But it is run from the offices of the local steelworkers union in rundown Gary, Indiana, and the Texas branch was founded in the headquarters of the American TUC. To not only create but cement an electorally viable political tradition, drawn from a mixture of the young, pacifists, trade unionists and environmentalists, in less than 20 years, is no mean feat. Keeping it together in government will be just as tough.

Key to its success will be the Greens' relationship both with the rest of the left and with its own activists. Nader, for example, is already being accused of drawing crucial support from Gore that might let Bush in through the back door. He rightly responds that the choice between Gore and Bush is scarcely a choice at all. Both are "drab and dreary" and reflect the interests of corporate America. If Gore wants his votes then he will have to shift to the left to get them, just as he has been shifting to the right to scoop up disenchanted Republicans.

The Greens in Germany have also made a difference. The agreement on nuclear power and the granting of citizenship rights for foreigners born in Germany have been major achievements which would not have happened without them. But they have paid heavily for their support and have been hopelessly compromised by Fischer's support for the war. While Nader may not wish to follow the German Greens, the German Greens, and their European and Antipodean allies, could do worse than learn some things from. He gained his bargaining position partly through his work in the anti-capitalist demonstrations in Seattle and Washington and joint campaigns with unions. He is the candidate for a party, but he is also part of a movement. His relative electoral strength is not an end in itself, but an illustration of his influence outside of the mainstream. The stronger he is from the outside, the more clout he will have on the inside.

Greens are right to take positions in government, but not at any price. Unlike their social democratic coalition partners, they are more than simply electoral machines. Taking up ministerial positions demands that they deepen, not weaken, their links with the wider movements. If the price for doing so is too high, they should forgo the opportunity and wait until the orthodox left has raised its game. It is possible to have power without office.